1-hexene and 1-octene are important commercial materials which are widely used as monomers or comonomers in polymerization processes to produce high value-added linear low-density polyethylene, as well as having end uses as specific drugs.
In this regard, higher alpha-olefin necessary for preparing high value-added linear low-density polyethylene is obtained via oligomerization of ethylene. However, ethylene oligomerization is inefficient in terms of producing considerable amounts of butene, other olefins and olefin isomers, specific higher oligomers, and polymers (polyethylene).
In conventional ethylene oligomerization techniques, the yield of a desired product is limited because a variety of alpha-olefins are produced depending on the Schulze-Flory or Poisson product distribution. In this regard, U.S. Pat. No. 6,184,428 discloses a nickel-based catalyst comprising 2-diphenyl phosphinobenzoic acid (DPPBA) as a chelate ligand, NiCl2.6H2O as a nickel precursor, and sodium tetraphenylborate as a catalyst activator, wherein the selectivity of 1-octene is reported to be 19% upon ethylene oligomerization using the same.
Also, German Patent No. 1,443,927 and U.S. Pat. No. 3,906,053 disclose a Ziegler-type catalyst based on a trialkylaluminum catalyst, through which about 13˜25 mass % of 1-octene may be produced from an olefin mixture.
Currently, research into selective tetramerization of ethylene using transition metal catalysis to produce 1-octene is ongoing, and most of the known transition metal catalysts are chromium-based catalysts.
In this regard, WO 02/04119 granted to BP Chemicals (whose current name is INEOS) discloses production of 1-hexene via highly active and highly selective trimerization of ethylene using chromium and diphosphine as a chelate ligand. Such a diphosphine ligand has a P—N—P backbone structure, wherein diphenyl coupled with each of phosphines contains a methoxy group substituted at the ortho position.
Also, WO 04/056478 and WO 04/056479 granted to Sasol Technology discloses tetramerization of ethylene with a selectivity of 70 mass % or more using a ligand having a P—N—P structure. The diphosphine/tetraphenyl-based ligand has a P—N—P backbone structure, but it requires the absence of a polar substituent at the ortho position of the phenyl group connected to a phosphine (P) atom. However, the ligand having a P—N—P structure enables partial trimerization, thus producing hexene, in which the amount of hexene, especially internal hexene, may increase in proportion to an increase in tetramer selectivity. Upon ethylene polymerization using a transition metal catalyst in the production process of linear low density polyethylene (LLDPE), internal hexene may act as a source for polluting the catalyst, and is thus regarded as an undesired byproduct. Although internal olefin may be removed up to a predetermined level via subsequent separation/purification, an increase in the selectivity of 1-hexene is basically favorable for commercialization.
U.S. Pat. No. 7,994,363 and US Patent Application No. 2011/0257352 granted to Nova Chemicals disclose an oligomerization process using an ethylene oligomerization catalyst in which fluorine is substituted at the ortho position of each of four phenyl groups coupled with two phosphines in a ligand limited only to a P—N—P backbone structure, but the oligomerization activity is considerably lower compared to when using a ligand having a P—C—C—P backbone structure as described later.
As alternatives to the P—N—P backbone structure mentioned in the prior patents, the present inventors proposed a method of producing 1-hexene and/or 1-octene from ethylene using a chromium-based catalyst system including a ligand having a P—C—C—P backbone structure in which two carbon atoms are interposed between phosphine (P) and phosphine (P) (Korean Unexamined Patent Publication Nos. 2008-0068226, 2009-0017929, 2010-0087913). Furthermore, in the case where the ligand having a P—C—C—P backbone structure is used, the catalytic activity is very stable during the reaction and thus the reaction rate may be continuously maintained. Also, compared to the conventional ligand having a P—N—P backbone structure, the ligand having a P—C—C—P backbone structure is advantageous because structures adjacent to the carbon atoms between two phosphine atoms thereof may be sterically arranged, and thus activity and selectivity of trimerization and tetramerization may be improved, and ethylene oligomerization activity may be greatly enhanced due to introduction of the ligand which is sterically asymmetric with respect to a plane.
Although the ligand having a P—C—C—P backbone structure has many advantages, there are still required techniques which enable a trimer and/or tetramer to be prepared with high activity and high selectivity, and also the production of internal olefin to be suppressed to thereby facilitate separation/purification.